The present invention relates to an apparatus for measuring the thickness of a film, and/or for monitoring the rate of increase of the thickness of a film, and to a method for carrying out such measuring and/or monitoring. In one aspect, the present invention relates to a quartz crystal thickness monitor which provides coating rate and thickness data in real time by monitoring change in frequency of vibration of a test crystal coated simultaneously with one or more process substrates, e.g., in the fabrication of optical devices (such as lenses, filters, reflectors and beam splitters) by optical thin-film deposition systems in which evaporant is deposited from deposition sources.
Since the early 1960""s, quartz crystals have been used to monitor thin film coating processes used in the fabrication of optical devices such as lenses, filters, reflectors and beam splitters. Although initially employed as an aid to optical monitors to provide information on the rate at which the film is deposited, quartz crystal sensors became relied upon to indicate and control optical layer thickness in automated deposition systems.
Research in fields such as nanotechnology, biosensors, thin film displays, and high-speed optical communications have increased the complexity of thin film structures. While an antireflection coating consisting of a single layer of magnesium fluoride may have been sufficient 20 years ago, current designs may call for a 24-layer stack of alternating refractive index films. With high-speed optical communications, this stack increases ten-fold, leading to filters comprised of up to 256 layers.
The manufacturing of these geometries requires the control and accuracy provided by a quartz crystal. Unfortunately, the materials and deposition temperatures utilized in today""s processing can adversely affect the operation of the crystal sensor.
Quartz crystal thickness monitors may be the most misunderstood components of optical thin film deposition systems. Quartz sensors provide process engineers with coating rate and thickness data in real time, with Angstrom resolution.
Quartz sensor instruments measure film thickness by monitoring a change in the frequency of vibration of a test crystal coated simultaneously with process substrates. Quartz is a piezoelectric material., i.e., if a bar of quartz is bent, it will develop a voltage on opposite faces. Conversely, if a voltage is applied, the bar will bend. By applying alternating voltage to such a bar, the bar will vibrate or oscillate in phase with the voltage.
At a specific frequency of oscillation, quartz will vibrate with minimal resistance, much like a tuning fork rings when struck. This natural resonance frequency is used as the basis for measuring film thickness. By adding coatings to the crystal surface, the resonance frequency decreases linearly. If the coatings are removed, the resonance frequency increases.
In a quartz crystal thickness monitor, the quartz crystal is coupled to an electrical circuit that causes the crystal to vibrate at its natural (or resonant) frequency, which for most commercial instruments is between 5 and 6 MHz. A microprocessor-based control unit monitors and displays this frequency, or derived quantities, continuously. As material coats the crystal during deposition, the resonant frequency decreases in a predictable fashion, proportional to the rate material arrives at the crystal, and the material density. The frequency change is calculated several times per second, converted in the microprocessor to Angstroms per second and displayed as deposition rate. The accumulated coating is displayed as total thickness.
The sensitivities of these sensors are remarkable. A uniform coating of as little as 10 Angstroms of aluminum will typically cause a frequency change of 20 Hz, easily measured by today""s electronics. As the density of the film increases, the frequency shift per Angstrom increases.
The useful life of quartz is dependent on the thickness and type of coating monitored. If a low stress metal such as aluminum is deposited, layers as thick as 1,000,000 Angstroms have been measured. At the other extreme, highly stressful dielectric films can cause crystal malfunction at thicknesses as low as 2,000 Angstroms or less.
In the early days of crystal thickness monitors, metallic films of copper, silver and gold were the materials that were deposited the most commonly. These films produced coatings of low stress and were condensed on substrates held near room temperature. Under these conditions, very accurate determinations of film thickness and rate were achievable.
When the optics industry began to employ crystal monitors, attention shifted from opaque metals to transparent materials such magnesium fluoride, and silicon dioxide, since coatings had to transmit light. Unfortunately, these substances produced films with high intrinsic stresses and required high process or substrate temperatures. These were not welcome developments for crystal monitoring, as sensors which have employed quartz have been highly sensitive to stress and temperature changes.
This sensitivity can be traced to the piezoelectric properties of quartz. Further complicating matters is the fact that quartz crystal sensors which have been employed have exhibited frequency change when deformed by thin film stresses or mechanical forces, e.g., from a mounting holder. If process conditions heat or cool such sensors, a similar frequency shift occurs. Regardless of the origin, the frequency shift is indistinguishable from that caused by the addition of coating.
Frequency shifts can be positive or negative, and can be cumulative. They can also be random. Causes of resonant frequency changes include:
Vibrations introduced through the mounting hardware;
Variations in the voltage used to oscillate the crystal;
Changes in the film being monitored (acoustic impedance);
Adhesion failure of the monitored coating or quartz electrodes; and
Radio frequency interference in the monitoring circuit.
These effects introduce large errors in thickness and rate calculations. Temperature swings in quartz can result in thickness variations of 50 Angstroms or more (see FIG. 1, which is a plot of frequency shift vs. temperature for AT-cut quartz crystal). Adhesion failure results in 100-Angstrom rate spikes. Extraneous vibrations can produce changes in the thousand Angstrom range. For precision optical components, these errors result in major yield loss.
The harsh conditions present during optical film coating can have deleterious effects on the operating life of a crystal. High stress coatings can deform the crystal to the point that it ceases to oscillate, without warning. Splatters of material from the coating source can lead to similar failure. High-energy plasmas used for substrate cleaning can couple into the crystal electronics and cause severe electrical noise. High temperature depositions can overheat the crystal, driving it past its operating limit.
Early crystal failure can be a great inconvenience or an unmitigated disaster. In the case of 100+ layer thin film stacks, venting the chamber to replace crystals is not an option, due to the undesirable effects of atmospheric gases on film chemistry. For very thick films, used in laser power or infrared optics, short crystal life may prevent completion of the coating. For high-speed roll coating systems, abrupt crystal failure can cause great amounts of ruined substrates.
Attempts have been made to reduce crystal failure and increase accuracy, e.g., through the use of sensors made of AT-cut quartz and through the use of water-cooled holders and/or sensor heads in order to maintain the temperature of the sensor between 20 degrees C. and 45 degrees C., in which temperature range the AT-cut quartz is xe2x80x9csubstantially temperature insensitive,xe2x80x9d (see FIG. 1) in order to reduce thermally induced frequency shifts for low temperature processes.
That is, in the past, films have been deposited at elevated temperatures in order to attempt to alleviate stresses which result from the films being built up. However, because such elevated temperatures cause the sensor to move out of the xe2x80x9csubstantially temperature-insensitive region,xe2x80x9d and result in frequency shifts in the thickness measurements of such sensor systems (see FIG. 1), prior systems have used cooling systems to try to counteract the effects of such heating, and to try to maintain the temperature in the substantially xe2x80x9ctemperature-insensitivexe2x80x9d region.
For example, conventional quartz crystal based thin film thickness sensor systems utilize a water cooled stainless steel holder which uses a thin (0.010xe2x80x3 thick) quartz crystal disk to measure the thickness, in situ and real time, of a thin film deposition process. This technology, available since the early 1960""s, is difficult to use when optical materials, such as magnesium fluoride, or silicon dioxide, are used in the coating process. These materials cause the crystal to act erratically and fail prematurely during the coating process, preventing the measurement and control function from taking place. It is thought that the intrinsic stresses that these materials have when deposited as thin films result in the quartz becoming strained microscopically. Typically, lenses to be coated are heated during coating to alleviate this stress.
The quartz sensor, placed near the structures (e.g., lenses) being deposited to monitor the process, has historically been water cooled at the same time, to minimize fluctuations in its reading due to temperature changes resulting from process heat (i.e., heat resulting from the process being used to deposit the coating). This cooling, unfortunately, compounds the stress problem on the crystal surface. Moreover, recent studies of standard sensor heads show that even with water-cooling, the crystal temperature can rise 20 to 30 degrees within a 10-minute process. For extended runs with high chamber temperatures, temperature increases can become considerably larger.
Others have attempted to generate temperature-frequency algorithms to try to cancel out the component of frequency change caused by temperature. Examples of such work include: (1) E. C. van Ballegooijen, xe2x80x9cSimultaneous Measurement of Mass and Temperature using Quartz Crystal Microbalancesxe2x80x9d Chapter 5, Methods and Phenomena 7, C. Lu and A. W. Czandema, Editors, Applications of Piezoelectric Quartz Crystal Microbalances, Elsevier Publishing, New York, 1984, and (2) E. P. Eernisse, xe2x80x9cVacuum Applications of Quartz Resonatorsxe2x80x9d, J. Vac. Sci. Technol., Vol. 12, No. 1, January/February 1975, pp 564-568.
A paradigm shift is underway in quartz crystal process monitoring. In many applications, crystals become the keys to success. No matter how significant a breakthrough may be in optics, be it materials, geometry, process design or application, if a thin film coating of any sophistication is required, the weak link is how accurately that film can be measured. As technology closes in on manipulating Angstrom-level properties of matter, the need for reliable thin film metrology rises to a new level of importance.
Film stress, adhesion failure, and extreme temperature effects have not been adequately dealt with. The current demands of nanotechnology, thin film displays, and high speed optical communications bring about an increased need for a quartz crystal monitor which reduces these inaccuracies and which reduces the frequency of such malfunctioning.
In accordance with a first aspect of the present invention, instead of trying to cool the piezoelectric element to counteract the effects of process heat being applied to the piezoelectric element, heat is directly applied to the piezoelectric element in order to heat the piezoelectric element to a temperature which is equal to or greater than the process conditions, such that stress is reduced, and even though the temperature is outside the xe2x80x9csubstantially temperature-insensitive range,xe2x80x9d because the temperature of the piezoelectric element is above the temperature of the processing, the temperature of the piezoelectric element can be maintained at a specific value, thereby eliminating any substantial frequency shift resulting from temperature variance.
According to this first aspect of the present invention, there is provided a device for measuring the thickness of a film and/or the rate of increase of the thickness of a film, the device comprising:
at least one piezoelectric element;
a first electrode, the first electrode being in contact with a first region of the piezoelectric element;
a second electrode, the second electrode being in contact with a second region of the piezoelectric element, the second region being spaced from the first region; and
a heater which heats the piezoelectric element.
Preferably, the heater heats the piezoelectric element to a temperature of at least about 50 degrees C., more preferably at least about 100 degrees C. The heater preferably maintains the piezoelectric element at a substantially constant temperature.
In accordance with a second aspect of the present invention, instead of the use of a sensor made of AT-cut quartz, the sensor is constructed of a different cut of quartz, namely, IT-cut quartz.
It has been surprisingly found that IT-cut quartz crystal provides performance superior to the industry standard AT-cut when used as a quartz crystal microbalance (i.e., thin film thickness sensor). The primary advantage to the sensor according to this aspect of the invention is the lack of substantial response to radiation induced frequency changes caused by heat sources or hot deposition sources present in a high vacuum thin film deposition system. When an AT-cut crystal in a conventional device is illuminated by a radiant source (such as a quartz lamp used to heat the substrates being coated), the sudden rise in temperature produces a sharp jump in oscillating frequency. This jump can be confused with frequency changes caused by the addition of mass to the crystal from the deposition source. Hence, an error in the accuracy of the film thickness is inadvertently introduced.
A second benefit of the sensor made of IT-cut quartz crystal according to this aspect of the invention is its diminished stress-frequency response. As an AT-cut crystal in a conventional device is deformed by the accumulation of high stress coatings (e.g., dielectrics used in optical coating processes), a frequency shift is introduced that, as in the radiation example, is indistinguishable from the frequency shift caused by mass accumulation. The IT-cut does not exhibit these frequency shifts to the degree of the AT-cut. Further, the useable life of an IT-cut quartz crystal microbalance is significantly longer than an AT-cut since stress-induced frequency noise does not obscure the mass-frequency behavior as readily.
According to this second aspect of the present invention, there is provided a device for measuring the thickness of a film and/or the rate of increase of the thickness of a film, the device comprising:
at least one piezoelectric element, the piezoelectric element comprising IT-cut quartz crystal;
a first electrode, the first electrode being in contact with a first region of the piezoelectric element; and
a second electrode, the second electrode being in contact with a second region of the piezoelectric element, the second region being spaced from the first region.
According to another aspect, the present invention is directed to a method of measuring the thickness of a film and/or the rate of increase of the thickness of a film, the method comprising:
applying a voltage across a piezoelectric element from a first electrode to a second electrode, thereby causing the piezoelectric element to vibrate, the first electrode being in contact with a first region of the piezoelectric element, the second electrode being in contact with a second region of the piezoelectric element;
applying heat to the piezoelectric element; and
measuring the rate of vibration of the piezoelectric element.
According to another aspect, the present invention is directed to a method of measuring the thickness of a film and/or the rate of increase of the thickness of a film, the method comprising:
applying a voltage across a piezoelectric element comprising IT-cut quartz crystal from a first electrode to a second electrode, thereby causing the piezoelectric element to vibrate, the first electrode being in contact with a first region of the piezoelectric element, the second electrode being in contact with a second region of the piezoelectric element; and
measuring the rate of vibration of the piezoelectric element.
The devices according to the present invention can be used to automatically control deposition sources, ensure repeatable and accurate thin film coatings, and control optical film properties dependent on deposition rate. The present invention provides improved accuracy.
The invention may be more fully understood with reference to the accompanying drawings and the following detailed description of the invention.